The knock at Hussam Wazzaz’s door came at around 6.30am. It was early, but Mr Wazzaz’s job as the head of the White Helmets rescue group in the Syrian town of Sarmin meant people often came at strange hours.

On the doorstep was a young man whose father was waiting for a White Helmets ambulance to take him to hospital in the nearby city of Idlib. The ambulance was supposed to have arrived 30 minutes ago but no one had showed up.

Mr Wazzaz glanced at his Whatsapp. The White Helmet volunteers on the overnight shift had not been online since 1am. Don’t worry, he told the young man, we’ll go check.

They climbed on a motorcycle and rode the short distance to the low-slung building the White Helmets used as an operation centre in Sarmin. The two ambulances normally parked outside were gone.

Mr Wazzaz pushed open the door of the centre. “The first thing I saw was the blood,” he said. “It was everywhere....